Core drilling with a diamond drill consists of rotating a tubular rod string and diamond bit at high speed (normally up to approx. 1800 rpm) and feeding a drill string into a rock formation. Diamonds impregnated in the matrix of the bit cut the rock resulting in a core entering the hollow drill rods as the bit advances. This core is retrieved by methods such as wirelining or reverse circulation, and retained for analysis. Water is pumped down the inside of the drill string (outside the drill string if using reverse circulation) in order to clear the cuttings from the bit, and keep the bit cool. The exposed diamonds on the bit wear and become dull, and thus require sharpening. Sharpening the bit is done in the hole and involves stripping a part of the matrix off the bit in order to expose new diamonds. This drilling and sharpening is a continual process until the bit is worn out and must be replaced. All the rods must be pulled out of the hole to replace the bit, and reinserted with a new bit.
Conventionally, diamond drilling has been done with manually operated hydraulic machines using a pressure controlled feed system. More recently, some attempts have been made to control the drilling process using microprocessors and electrically controlled actuators. To our knowledge, all of these attempts have involved using electric actuators to replace the handle on otherwise manually controlled pressure control valves. This approach may not allow a microprocessor to be used to its full potential in the control of the drilling process. Some of the most important inherent difficulties in these systems include poor positioning accuracy and poor response time to changing load conditions in the hole, as would occur when penetrating through rock formation transitions or faults, and sharpening the bit, particularly in deep hole conditions.
U.S. Pat. No. 4,157,231 discloses a hydraulic drilling unit for precision machine drilling, which uses a modulating servo valve to adjust a feed rate under computer control, but in the positive direction only. Because this patent does not provide a holdback force to compensate for the length and weight of a diamond drill string, it can not be applied to a diamond drill.
U.S. Pat. No. 5,449,047 discloses a computer control system for blast hole drilling, which also uses unidirectional modulation. Again, because it does not provide a holdback force to compensate for the length and weight of drill string, it is not appropriate for a diamond drill. Further, this patent teaches that the drill rate of penetration should be reduced when passing through less dense material or voids.
Germany Patent No. 94 02 360 discloses a computer control system for auger drilling, which also uses unidirectional modulation. Again, because it does not provide a holdback force to compensate for the length and weight of drill string, it is not appropriate for a diamond drill.